Today's high-performance microprocessors and microcomputers place ever increasing demands on the development system tools used for debugging these sophisticated integrated circuits. Moreover, the increasing variety of standard and custom/core microcomputer (MCU) designs, necessitates a fundamental change in the design of development system tools. New microprocessors (MPU), designed to operate at clock speeds upwards of 33 MHz, push development system technology to its limit. Meanwhile, the widespread usage of existing central processing unit (CPU) designs for the processor core in MCUs imposes constraints on the design of development system tools. As the design cycle for these standard and custom/core MCUs shortens, concurrent availability of development system tools becomes increasingly problematic. Consequently, the problem is one of timely availability of high performance development system tools to support the wide variety of microprocessors and microcomputers entering the marketplace.
The traditional design approach for development system support relies upon in-circuit emulation to debug the MCU/MPU prototype software/hardware. Typically, in-circuit emulation allows the software designer to trace the execution of the software by stopping the program execution, at various intervals, and examining the contents of the internal registers, memory locations, and the like. In some designs the in-circuit emulator operates in a foreground and background mode, whereby the emulator processes the application program (non-emulation) in the foreground mode and the debugging program (emulation) in the background mode. Accordingly, the in-circuit emulator switches banks between foreground and background memory space during the execution and debugging of the user program, therefore, the MCU must operate in an expanded mode, necessitating the reconstruction of I/O ports which are lost to the external bus interface. Generally, the switch back and forth between the foreground (user) and background (debugging) memory banks requires substantial amounts of external buffer circuitry, to facilitate the operation of the MCU in an expanded mode. Often this additional buffer circuitry degrades the performance of the emulator, and thus prevents real-time circuit emulation. Since the implementation of the foreground/background memory bank switch requires a substantial amount of external circuitry, an emulator pod is used rather than a small daughter-board probe. The design of the resultant emulator pod is time consuming, and costly due to the required development system engineering expenses.
In the prior art, system designers employed port-replacement devices or bond out devices, in an effort to reduce the amount of external circuitry required. The primary disadvantage of using a port-replacement or bond out device is the requirement for additional design and maintenance resources. Essentially, the system designer must allocate design resources for the port-replace or bond out device, in addition to those resources required for the MCU/MPU design. Yet another approach, disclosed in Vaglica et al. U.S. Ser. No. 115,479 filed Oct. 30, 1987, is to integrate a background mode into the CPU core. This solution is optimal in some cases, however, it is often infeasible to make anything other than minor modifications to existing CPU cores.